HTTPbis M. West
Internet-Draft Google, Inc
Updates: 6265 (if approved) October 27, 2014
Intended status: Standards Track
Expires: April 30, 2015
First-Party Cookies
draft-west-first-party-cookies-00
Abstract
This document updates RFC6265, defining the "First-Party" attribute
for cookies, which allows servers to mitigate the risk of cross-site
request forgery and related information leakage attacks by asserting
that a particular cookie should only be sent in a "first-party"
context.
Status of This Memo
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This Internet-Draft will expire on April 30, 2015.
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Copyright (c) 2014 IETF Trust and the persons identified as the
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and notation . . . . . . . . . . . . . . . . . . 3
2.1. First-party and Third-party Requests . . . . . . . . . . 3
3. Server Requirements . . . . . . . . . . . . . . . . . . . . . 4
3.1. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Semantics of the "First-Party" Attribute (Non-Normative) 4
4. User Agent Requirements . . . . . . . . . . . . . . . . . . . 5
4.1. The "First-Party" attribute . . . . . . . . . . . . . . . 5
4.2. Monkey-patching the Storage Model . . . . . . . . . . . . 5
4.3. Monkey-patching the "Cookie" header . . . . . . . . . . . 6
5. Authoring Considerations . . . . . . . . . . . . . . . . . . 6
5.1. Mashups and Widgets . . . . . . . . . . . . . . . . . . . 6
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6
6.1. User Controls . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7.1. Limitations . . . . . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Section 8.2 of [RFC6265] eloquently notes that cookies are a form of
ambient authority, attached by default to requests the user agent
sends on a user's behalf. Even when an attacker doesn't know the
contents of a user's cookies, she can still execute commands on the
user's behalf (and with the user's authority) by asking the user
agent to send HTTP requests to unwary servers.
Here, we update [RFC6265] with a simple mitigation strategy that
allows servers to declare certain cookies as "First-party cookies"
which should be attached to requests if and only if they occur in a
first-party context.
Note that the mechanism outlined here is backwards compatible with
the existing cookie syntax. Servers may serve first-party cookies to
all user agents; those that do not support the "First-Party"
attribute will simply store a non-first-party cookie, just as they do
today.
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1.1. Examples
First-party cookies are set via the "First-Party" attribute in the
"Set-Cookie" header field. That is, given a server's response to a
user agent which contains the following header field:
Set-Cookie: SID=31d4d96e407aad42; First-Party
Subsequent requests from that user agent can be expected to contain
the following header field if and only if both the requested resource
and the resource in the top-level browsing context match the cookie.
2. Terminology and notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234].
Two sequences of octets are said to case-insensitively match each
other if and only if they are equivalent under the "i;ascii-casemap"
collation defined in [RFC4790].
The terms "active document", and "top-level browsing context" are
defined in the HTML Living Standard. [HTML]
The term "origin" and the mechanism of deriving an origin from a URI
are defined in [RFC6454].
2.1. First-party and Third-party Requests
The URL displayed in a user agent's address bar is the only security
context directly exposed to users, and therefore the only signal
users can reasonably rely upon to determine who they're talking to.
Broadly speaking, then, a "first-party" request is an HTTP request
for a resource whose URL's origin matches the origin of the URL the
user sees in the address bar. A "third-party" request is an HTTP
request for a resource at any other origin.
To be slightly more precise, given an HTTP request "request":
1. Let "context" be the top-level browsing context in the window
responsible for "request".
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2. Let "top-origin" be the origin of the location of the active
document in "context".
3. If the origin of "request"'s URL is the same as "top-origin",
"request" is a *first-party request*. Otherwise, "request" is a
*third-party request*.
Note that we deal with the document's _location_ in step 2 above, not
with the document's origin. For example, a top-level document from
"https://example.com" which has been sandboxed into a unique origin
still creates a non-unique first-party context for subsequent
requests.
This definition has a few implications:
o New windows create new first-party contexts.
o Full-page navigations create new first-party contexts. Notably,
this includes both HTTP and ""-driven redirects.
o ""s do _not_ create new first-party contexts; their
requests MUST be considered in the context of the origin of the
URL the user actually sees in the user agent's address bar.
3. Server Requirements
This section describes extensions to [RFC6265] necessary to implement
the server-side requirements of the "First-Party" attribute.
3.1. Grammar
Add "First-Party" to the list of accepted attributes in the "Set-
Cookie" header field's value by replacing the "cookie-av" token
definition in Section 4.1.1 of [RFC6265] with the following ABNF
grammar:
cookie-av = expires-av / max-age-av / domain-av /
path-av / secure-av / httponly-av /
first-party-av / extension-av
first-party-av = "First-Party"
3.2. Semantics of the "First-Party" Attribute (Non-Normative)
The "First-Party" attribute limits the scope of the cookie such that
it will only be attached to requests if those requests are "first-
party", as described in Section 2.1. For example, requests for
"https://example.com/sekrit-image" will attach first-party cookies if
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and only if the top-level browsing context is currently displaying a
document from "https://example.com".
The changes to the "Cookie" header field suggested in Section 4.3
provide additional detail.
4. User Agent Requirements
This section describes extensions to [RFC6265] necessary in order to
implement the client-side requirements of the "First-Party"
attribute.
4.1. The "First-Party" attribute
The following attribute definition should be considered part of the
the "Set-Cookie" algorithm as described in Section 5.2 of [RFC6265]:
If the attribute-name case-insensitively matches the string "First-
Party", the user agent MUST append an attribute to the "cookie-
attribute-list" with an "attribute-name" of "First-Party" and an
empty "attribute-value".
4.2. Monkey-patching the Storage Model
Note: There's got to be a better way to specify this. Until I figure
out what that is, monkey-patching!
Alter Section 5.3 of [RFC6265] as follows:
1. Add "firstparty-flag" to the list of fields stored for each
cookie.
2. Before step 11 of the current algorithm, add the following:
1. If the "cookie-attribute-list" contains an attribute with an
"attribute-name" of "First-Party", set the cookie's "first-
party-flag" to true. Otherwise, set the cookie's "first-
party-flag" to false.
2. If the cookie's "first-party-flag" is set to true, and the
request which generated the cookie is not a first-party
request (as defined in Section 2.1), then abort these steps
and ignore the newly created cookie entirely.
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4.3. Monkey-patching the "Cookie" header
Note: There's got to be a better way to specify this. Until I figure
out what that is, monkey-patching!
Alter Section 5.4 of [RFC6265] as follows:
1. Add the following requirement to the list in step 1:
* If the cookie's "first-party-flag" is true, then exclude the
cookie if the HTTP request is a third-party request (see
Section 2.1).
Note that the modifications suggested here concern themselves only
with the origin of the top-level browsing context and the origin of
the resource being requested. The cookie's "domain", "path", and
"secure" attributes do not come into play for this comparison.
5. Authoring Considerations
5.1. Mashups and Widgets
The "First-Party" attribute is inappropriate for some important use-
cases. In particular, note that content intended for embedding in a
third-party context (social networking widgets or commenting
services, for instance) will not have access to first-party cookies.
Non-first-party cookies may be required in order to provide seamless
functionality that relies on a user's state.
Likewise, some forms of Single-Sign On might require authentication
in a third-party context; these mechanisms will not function as
intended with first-party cookies.
6. Privacy Considerations
6.1. User Controls
First-party cookies in and of themselves don't do anything to address
the general privacy concerns outlined in Section 7.1 of [RFC6265].
The attribute is set by the server, and serves to mitigate the risk
of certain kinds of attacks that the server is worried about. The
user is not involved in this decision.
User agents, however, could offer users the ability to toggle a
cookie's "first-party-flag" themselves, perhaps as part of a more
general cookie management interface. This could provide an
interesting middle-ground between the options (e.g. "Block all",
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"Block third-party", and "Allow all") that many user agents offer to
users.
7. Security Considerations
7.1. Limitations
It is possible to bypass the protection that first-party cookies
offer against cross-site request forgery attacks by creating first-
party contexts in which to execute the attack. Consider, for
instance, the URL "https://example.com/logout" which logs the current
user out of "example.com". If the user's session cookie is first-
party cookie, then embedding the logout URL in an "" element
or an "" element won't log her out, as the cookie won't be sent.
Popping up a new window, or doing a top-level navigation, on the
other hand, will create a first-party context, attach cookies, and
perform the logout.
Note, though, that popping up a window, or doing a top-level
navigation are both significantly more visible to the user than
loading a subresource. Users will at least have the opportunity to
notice that something strange is going on, which hopefully reduces an
attacker's ability to perform untargeted attacks.
Further, note that certain kinds of attacks are no longer possible if
a first-party context is required. Information leakage attacks which
rely on visible side-effects of loading a session-protected image,
for example, can no longer access those side-effects if the image is
loaded in a new window. Timing attacks like those Paul Stone
outlines in [pixel-perfect] are no longer possible if the session
cookie is first-party, as they rely on "" to contain the
protected content in a way the attacker can manipulate.
8. Acknowledgements
The first-party cookie concept documented here is similar to (but
stricter than) Mark Goodwin's and Joe Walker's [samedomain-cookies].
9. References
9.1. Normative References
[HTML] Hickson, I., "HTML Living Standard", n.d.,
.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC4790] Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
Application Protocol Collation Registry", RFC 4790, March
2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, December
2011.
9.2. Informative References
[pixel-perfect]
Stone, P., "Pixel Perfect Timing Attacks with HTML5",
n.d., .
[samedomain-cookies]
Goodwin,, M. and J. Walker, "SameDomain Cookie Flag",
2011, .
Author's Address
Mike West
Google, Inc
Email: mkwst@google.com
URI: https://mikewest.org/
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